Hot-wire type air flow meter and an internal combustion engine with the same

ABSTRACT

A compact hot-wire type air flow meter for an internal combustion engine with a high measuring accuracy is provided with a primary flow path forming an intake air passage and an auxiliary flow path incorporating therein a hot-wire element for measuring the intake air. The auxiliary flow path is defined by a flow path in an axial direction of the primary flow path and a flow path in a radial direction of the primary flow path.

BACKGROUND OF THE INVENTION

The present invention relates to a hot-wire type air flow meter, andmore particularly to a hot-wire type air flow meter for an automotiveinternal combustion engine, which constitutes an intake system of theinternal combustion engine, and is adapted to detect and control theflow rate of intake air.

As shown in, for example, Japanese Utility Model Unexamined PublicationNo. 56-135127 and Japanese Patent Unexamined Publication No. 60-185118,there has been provided a conventional passage structure for a hot-wiretype air flow meter for an automotive engine, in which an auxiliary flowpath is formed in an intake pipe; a hot-wire element is arranged in theauxiliary flow path; an obstacle or a complicated bent flow path that islong in the axial direction is provided downstream of the hot-wireelement for the purpose of protecting the hot-wire element against abackfire or a backblow of the engine and for the purpose of preventingan abnormal output of the hot-wire element caused by the pulsation ofthe engine. In such a flow meter, since the auxiliary path portionincluding the hot-wire element is formed in such manner as to exposed tothe primary flow, an output error caused by the temperature increase ofthe flow meter body is small. However, this arrangement requires a longphysical length in the axial direction and a large number of mechanicalparts which are difficult to mount. Therefore, this arrangement suffersfrom defects in compactness and cost.

Also, as disclosed in, for example, Japanese Patent UnexaminedPublication Nos. 57-23818 and 57-113926, there has been proposed anarrangement in which a hot-wire type air flow meter and a throttle valvemeans are disposed close to each other in an integral body. In JapanesePatent Unexamined Publication No. 57-23818, the same techniques as thosein the foregoing two publications are adopted in the arrangement inwhich the auxiliary passage within which the hot-wire element isdisposed is defined by a straight pipe and is formed in the centralportion of the primary passage. However, in the publication '818, thereis no protection for the hot-wire element against backfire and backblowof the engine. The throttle valve downstream of the primary flow mightserve as a protection means under the condition where it is almostclosed, but the throttle valve will have no use as protection meansunder the full or almost full open condition thereof. Also, in additionto this problem, this arrangement suffers from another problem in whichthe flow within the auxiliary flow path tends to become unstable inresponse to the movement of the throttle valve. Japanese PatentUnexamined Publication No. 57-113926 discloses an auxiliary flow path inwhich a hot-wire element is disposed within a body wall having a largethermal capacity and having no wide relative transfer area, saidauxiliary flow path having an L-shape formed by a first flow pathparallel to a primary flow and a second flow path perpendicular to thefirst flow path. With such an arrangement, it is possible to protect thehot-wire element against blowback or backfire of the engine. However,due to the structure of the auxiliary flow path, since air of theprimary flow cannot flow around the auxiliary flow path wall, thetemperature of the auxiliary flow path wall is highly increased due tothe heat generated by the hot-wire element as well as the heattransferred from the engine. As a result, the air within the auxiliaryflow path is heated so that the difference in temperature between theair in the auxiliary flow path and the air in the primary flow path islarge. Thus, it is impossible to exactly measure the flow rate of theintake air.

The foregoing prior art is silent with respect to the need for reductionof the pipe length between the hot-wire type air flow meter and thethrottle valve means. Therefore, the prior art suffers from the problemsof increase of pressure loss in the intake passage and of increase ofweight and cost of the equipment. Moreover, the prior art encounters thefollowing difficulties: (1) a heat generation of the hot-wire element;(2) a temperature increase of the auxiliary flow path wall around thehot-wire element due to thermal invasion from the outside, that is, anerror due to a difference between a temperature of the actual intake airand the temperature of the air flowing through the auxiliary flow pathwhile impinging against the hot-wire element and a temperaturecompensation element; (3) a countermeasure against a change of the flowrate distribution ratio between the primary flow path and the auxiliaryflow path due to the swirl or change of the intake air or the change offlow downstream of the flow meter, even if the constant distribution isintended; a reduction of flow turbulence within the auxiliary flow path,that is, the reduction of the output noises; (4) a protection for theelements against the counterflow due to the backblow or backfire and thepulsation; and (5) a countermeasure against abnormal output.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a hot-wiretype air flow meter which is compact with a high measuring precision.

Another object of the present invention is to provide an internalcombustion engine which is capable of controlling an air/fuel ratio in asuitable manner with use of the above-described hot-wire type air flowmeter.

In order to attain these and other objects, according to the presentinvention, there is provided a hot-wire type air flow meter comprising aprimary flow path constituting an intake air passage of an internalcombustion engine, a hot-wire element for measuring an intake air, andan auxiliary flow path provided in the primary flow path, having thereinsaid hot-wire element, the auxiliary flow path being defined by a flowpath formed in an axial direction of the primary flow path and flowpaths formed in a radial direction of the primary flow path.

According to the present invention, there is provided an internalcombustion engine comprising the above-described hot-wire type air flowmeter, a speed sensor for detecting an rpm of the engine, a fuelinjection means for injecting fuel into an intake air, and a controlmeans for determining a fuel injection amount based upon the rpmdetected by the speed sensor and a flow rate of the intake air detectedby the hot-wire type air flow meter, and for outputting a command signalfor injecting the determined fuel injection amount to said fuelinjection means.

According to the present invention, a hot-wire element is disposed in anauxiliary flow path independent of a primary flow path, thereby reducingan adverse effect of turbulence in the primary flow path. Also, in theauxiliary flow path having a small diameter relative to that of theprimary flow path, a distance between the auxiliary flow path inlet andthe hot-wire element is twice longer than the diameter of the auxiliaryflow path or more, thereby performing a rather rectification of the flowto reduce the noises. Also, the auxiliary flow path downstream of theelement is bent, so that the flow at the bent portion and the pressuredamping effect prevent a damage of the hot-wire element due to thecounter flow and reduce the adverse effect of the pulsation. Accordingto the present invention, the distance from the inlet of the auxiliaryflow path to the element is twice longer than the diameter of theauxiliary flow path or more, and the inlet of the auxiliary flow path isconstructed so that it projects into the primary flow path at a constantdistance from a body inner wall or a portion connecting the body innerwall and the auxiliary flow path. Furthermore, a bent auxiliary flowpath having a flow path wall of the projecting portion wall has a shortaxial length downstream of the element and may be coupled substantiallydirectly to a throttle valve means, to thereby solve various problemsdue to noises, pulsation and counter flows.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a cross-sectional view showing one embodiment of theinvention;

FIG. 2 is a cross-sectional view taken along the line II--II of FIG. 1;

FIG. 3 is a cross-sectional view taken along the line III--III;

FIG. 4 is a cross-sectional view showing another embodiment of theinvention;

FIG. 5 is a cross-sectional view taken along the line V--V of FIG. 4;

FIG. 6 is a cross-sectional view taken along the line VI--VI of FIG. 4;

FIG. 7 is a cross-sectional view showing another embodiment of theinvention;

FIG. 8 is a cross-sectional view taken along the line VIII--VIII of FIG.7;

FIG. 9 is a cross-sectional view showing another embodiment of theinvention;

FIG. 10 is a cross-sectional view taken along the line X--X of FIG. 9;

FIG. 11 is a cross-sectional view showing another embodiment of theinvention;

FIG. 12 is a cross-sectional view taken along the line XII--XII of FIG.11;

FIG. 13 is a cross-sectional view showing another embodiment of theinvention;

FIG. 14 is a cross-sectional view taken along the line XIV--XIV of FIG.13;

FIG. 15 is a cross-sectional view taken along the line XV--XV of FIG.13;

FIG. 16 is a cross-sectional view showing a part of an auxiliary flowpath according to an embodiment of the invention;

FIG. 17 is a cross-sectional view taken along the line XVII--XVII ofFIG. 16;

FIG. 18 is a cross-sectional view showing a part of an auxiliary flowpath according to another embodiment of the invention;

FIG. 19 is a cross-sectional view taken along the line XIX--XIX of FIG.18;

FIG. 20 is a cross-sectional view showing another embodiment of theinvention;

FIG. 21 is a cross-sectional view taken along the line XXI--XXI of FIG.20;

FIG. 22 is a cross-sectional view showing another embodiment of theinvention;

FIG. 23 is a cross-sectional view taken along the line XXIII--XXIII ofFIG. 22;

FIGS. 24, 25, 26 and 27 are cross-sectional views showing otherembodiments of the invention;

FIG. 28 is a cross-sectional view taken along the line XXVIII--XXVIII ofFIG. 27;

FIG. 29 shows a modification of a part shown in FIG. 27; and

FIG. 30 is a view showing a system of an electronic fuel injection meansaccording to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of the present invention will now be described withreference to FIGS. 1, 2 and 3. A body 1 constitutes an intake passage ofan internal combustion engine. Intake air is introduced from the leftside in FIG. 1. The internal combustion engine is connected on thedownstream side of the flow.

The body 1 forms a substantially cylindrical primary flow path 3. Aprojecting portion 2 that is formed integrally with the body 1 isdisposed in the primary flow path 3. At a tip end of the projectingportion 2, there is provided an auxiliary flow path 4 that is parallelto the primary flow path 3 and has an inlet opening at the centralportion of the primary flow path 3. Also, a hole in communication withthe outside of the body 1 is formed in the projecting portion 2. In thathole, there is received a mold unit 13 of a support member 11 for ahot-wire element 10 connected to a circuit unit 14. As a result, thehot-wire element 10 and a temperature compensation element 12 aredisposed in the auxiliary flow path 4. Downstream of the auxiliary flowpath 4, a bent auxiliary flow path 5 having a short axial length isformed by walls 2a, 2b and 2c of the projecting portion 2 and a cover 6.A throttle valve 20 for controlling a whole air flow rate is disposed atthe downstream side of the bent auxiliary flow path 5. The throttlevalve 20 is opened/closed by a valve drive shaft 21. A link mechanism(not shown) connected to the shaft 21 is provided outside the body 1.The link mechnism is normally driven by a cable connected to anaccelerator pedal of the vehicle. Incidentally, the cover 6 is mountedat a rear end of the projecting portion 2 by bolts 7 and 8 prior to themounting of the throttle valve 20 and the valve drive shaft 21.

An opening portion 4a of the auxiliary flow path 4 is mounted so that itis separated by a distance that is twice as long as the inner diameterof the auxiliary flow path 4 or more, from an inner wall 1a of the body1 and from a wall 2d of a connecting portion between the projectingportion 2 and the body 1. Also, the opening portion 4a is in the form ofa bellmouth.

An inner wall 1a of the body 1 and an outer wall 2e or the like, formingthe auxiliary flow path 4 of the projecting portion 2 are configured sothat the flow path is expended toward the upstream side. On the otherhand, an inner wall 1b of the body 1 at which the throttle valve 20 isprovided is finished by machining in such manner is to have the samediameter as other's. Before the machining work, the wall 1b is in theform of a cone converging toward the leftside of FIG. 1. With suchtechnique, the flow path may be cast-molded by using removable coremolds having a division plane in the vicinity of the projecting portionwall 2a. The core molds may be removed in the right and left directions.

The blank arrows indicate the flow of air. Although almost of the airentering from the left hand in FIG. 1 will flow through the primary flowpath 3, a part of the air will be introduced into the auxiliary flowpath 4. Since the inlet 4a of the auxiliary flow path is sufficientlyseparated from the walls 1a and 2d, the flow having a relatively lowturbulence is introduced into the auxiliary flow path 4.

Also, the bellmouth of the inlet 4a of the auxiliary flow path 4a mayentrain a large amount of air to thereby increase the air flow speed inthe vicinity of the inner wall 2f of the auxiliary flow path 4. However,the friction effect of the inner wall 2f of the auxiliary flow path 4 upto the hot-wire element causes the flow within the auxiliary flow path 4to be sufficiently rectified so that the flow immediately before thehot-wire element 10 has a uniform flow speed distribution.

A ratio of an inlet diameter of the bellmouth to the diameter of theauxiliary flow path 4 is in the range of 1.6 to 1.2. Correspondingly, aratio of the distance from the inlet to the hot-wire element 10 to thediameter of the auxiliary flow path 4 is in the range from about 4 to 2.However, this relation is changed in accordance with actual dimensionsof the diameter of the auxiliary flow path 4 but is not a so-calledone-to-one relation.

In the downstream of the hot-wire element 10, the flow is curvedupwardly to be introduced into the bent auxiliary flow path 5, andsubsequently is impinged against the inner wall of the body to flow outto the right and left through the outlets 5a and 5b to be immerged intothe main flow. Such a flow path arrangement has an effect to damp thecounter flow from the engine and to prevent the propagation of thepulsation to the vicinity of the hot-wire element 10.

According to the foregoing embodiment, with a short axial dimension, itis possible to provide a hot-wire type air flow meter for an internalcombustion engine, which is free from various problems such that noisesare made due to the turbulence of flow, unstable outputs are generateddue to the affect of the pulsation, and the hot-wire element is damageddue to the backblow of the engine. Namely, the advantages of thecompactness, the lightweight and the low cost may be enjoyed. The flowmeter body and the throttle valve unit body that cannot be formedintegrally in the prior art may be formed in a single integral bodyunit. Since the length of the intake passage is reduced, the arrangementis available in reduction in unduly pressure loss, lightweight and lowcost.

FIGS. 4 to 6 show a second embodiment of the invention. The differenceover the first embodiment shown in FIGS. 1 to 3 will be explained. Aprojecting portion 42 is formed integrally with the upper and lowerwalls of a body 41 (or is integral with the right and left walls of thebody, i.e., in the axial direction of the throttle valve drive shaft 21,if desired). With such an arrangement, it is possible to form bentauxiliary flow paths 45 downstream of the auxiliary flow path 44 in theup and down directions. Also, the outlet surface of the auxiliary flowpath 44, i.e., a rear end face 42a of the projecting portion 42 is madein a flat surface. This makes it easy to perform a machining workbecause of the reduction of the surface roughness. Also, an auxiliaryflow path cover 46 is formed in U-shape in cross section unlike thesimple planar plate as in the first embodiment. The cover 46 is alsomounted on the rear end face of the projecting portion 42 by means ofbolts 7 and 8. Since the auxiliary flow path 46 is made of a memberseparated from the body, it is possible to finish the rear end face 42aof the projecting portion 42. Therefore, it is possible to reduce thesurface roughness of the inner surface of the bent auxiliary flow paths45 as a whole. Also, it is possible to obtain a good sealing effect onthe connecting portions. This is effective to avoid the case where theperformance of the hot wire element 10 is unstable due to theunstability of the flow within the bent auxiliary flow paths and theinsufficiency of the pressure seal against the primary flow path.

It is apparent that, in the first embodiment, the same effect as in thesecond embodiment may be ensured by making the rear end face of theprojecting portion 2 flat and using the U-shaped cover instead of theplanar cover 6.

The effect of the second embodiment in which the bent auxiliary flowpaths 45 are provided in the up and down direction is the interferenceeffect in front of the auxiliary flow path 44 in the case where thepulsation is produced. Namely, the second embodiment is more availableagainst the pulsation. However, in the second embodiment, since the flowpath resistance is decreased, it is desired that some modification suchas reduction of the area of the outlets 45a to 45d be made in conformitywith the engine.

There is no difference in structural effect between the first and secondembodiments.

FIGS. 7 and 8 show a third embodiment of the invention. An auxiliaryflow path 74 in parallel to a primary flow path 73 is provided at aportion near to the inner wall of the body 71 rather than the tip end ofthe projecting portion 72 from the body 71. The bent auxiliary flow path75 downstream of the auxiliary flow path 74 is defined by a rear endwall 72a of the projecting portion 72 and an auxiliary flow path cover76 mounted on the wall 72a by a bolt 7. The rear end portion of theprojecting portion 72 extends close to the central portion of theprimary flow path 73. Therefore, the flow within the bent auxiliary flowpath 75 is first directed from the inner circumferential wall of thebody 71 along the wall 72a toward the central portion of the primaryflow path 73. Then, the air is made to flow from the bent auxiliary flowpath outlet 75a toward the right and left and downwardly. Downstream ofthe bent path 75, the throttle valve 20 and the drive shaft 21 aredisposed within the integral body in the same manner as in the first andsecond embodiments.

According to the features of the third embodiment, it is possible toreduce a length of a molded portion 83 integral with the circuit unit84, which is available in cost. Also, since the projecting portion 72may be relatively short, it is possible to reduce the flow resistance ofthe primary flow path 73. Also, since the mass corresponding to theoverhang portion is small, the structure is advantageous against thevibration in comparison with the first embodiment. However, theturbulence in the flow entering into the auxiliary flow path 74 issomewhat larger. Therefore, it is desired that, for that reason, thediameter of the bellmouth be enlarged and the distance to the hot-wireelement 10 of the auxiliary flow path 74 be elongated.

The basic effect of the third embodiment is substantially the same asthat of the first embodiment.

FIGS. 9 and 10 shows a fourth embodiment of the invention.

Unlike the first through third embodiments, a body 91 is made of asingle unit of a hot-wire type flow meter. A projecting portion 92 isformed substantially in the same manner as the first embodiment. Anauxiliary flow path 94 is formed at an end of the projecting portion 92.A part of the bent auxiliary flow path 95 is formed along a rear endface 92a of the projecting portion 92. The rest of the bent auxiliaryflow path 95 is formed so as to project outside the primary flow path 93and to enter the body 91 to be branched into the right and left sidesfrom the upper side in the range of about 90 degrees as shown in FIG.10. Therefore, the outlets of the bent auxiliary flow path 95 arelocated on both sides as shown in FIG. 10. The flow path surface of theportion downstream of the bent auxiliary flow path 95 is formed by agasket 96. Namely, a body of the throttle valve unit independent of theflow meter body 91 is coupled through the gasket to the flow meter bodywith bolt holes 98 a to 98d.

Since the bent auxiliary flow path 95 may be elongated, the fourthembodiment may be applied to an engine which suffers from a largepulsation.

FIGS. 11 and 12 show a fifth embodiment of the invention, in which areinforcement rib 8 is added to the structure shown in the firstembodiment. More specifically, there are provided a part for forming theauxiliary flow path 4 at the tip end of the projecting portion 2 of thefirst embodiment and a rib 8 connected to an opposite inner wall of thebody 1. With such an arrangement, it is possible to increase a strengthfor an earthquake-proof and reduce a deformation of the projectingportion 2 during the cast molding. The other effects are the same asthose of the first embodiment.

FIGS. 13 to 15 show a sixth embodiment of the invention. A projectingportion 132 from the body 131 is formed by ribs 137 and 138 formed in adirection perpendicular to the mold portion 13 connected to the circuitunit 14 and a cylindrical portion 132 defining the auxiliary flow path134. Therefore, the molded portion 13 of the circuit unit 14 is passedthrough a wall of the body 131 and once crosses the primary flow path133 to penetrate a hole of the projecting portion 132, so that the hotelement 10 is disposed within the auxiliary flow path 134. An O-ring 139is disposed at a portion of the molded portion 13 inserted into the holeof the projecting portion 132. The O-ring 139 serves to impart the sealeffect between the primary flow path 133 and the auxiliary flow path134. A bent auxiliary flow path 135 is defined by a rear end face of theprojecting portion 132 and a cover 136. Two outlets 135a and 135b areformed in the up and down direction of the cover 136. The outlets 135aand 136b are formed so that the flow therethrough is rather returnedback to the upstream side. This is because the length of the bentauxiliary flow path is short and the orientation of the outlets maycompensate for this shortage.

The advantage of this embodiment is that, since the projecting portions137, 138 and 132 are located in the direction of the throttle valvedrive shaft 21 that is inherently an obstacle or block against theprimary flow path 133, it is possible to reduce the substantial flowresistance within the primary flow path 133. Also, in this embodiment,the inlet portion 133a of the primary flow path 133 is in the form of abellmouth to thereby impart the rectifying effect.

The various embodiments of the invention have been described but it isapparent that in any of the embodiments, the cover member of the bentauxiliary flow path is not necessarily mounted by bolts. Any othermounting means such as bonding or adhesive may be used and it ispossible to seal the contact portion of the projecting portion rear faceand the cover with seal material.

FIGS. 16 and 17 show a seventh embodiment of the invention which issubstantially the same structure as that of the sixth embodiment.However, in the seventh embodiment, flow paths 140a, 140b or the likeperpendicular to the primary flow are formed in the cover 139. Accordingto this embodiment, since a flow path cross section of each of the flowpath 140a, 140b may be reduced, it is possible to further reduce theaxial length.

FIGS. 18 and 19 show an eighth embodiment of the invention which issubstantially the same structure as that of the sixth embodiment.However, in the eighth embodiment, the flow path 142 perpendicular tothe primary flow is in the form of a disc, that is, if the bypass flowpath 134 is included, the flow path 142 is in the form of a mushroom.According to the present embodiment, it is possible to reduce the axiallength in comparison with the seventh embodiment.

FIGS. 20 and 21 show a ninth embodiment of the invention. The projectionportion 210d, provided with the auxiliary flow path 212, which isintegral with the body 210 and projected into the primary flow path 211is formed through about 90 degrees along the inner wall of the body.Therefore, the auxiliary flow path 212c perpendicular to the auxiliaryflow path 212b in parallel with the primary flow path 211 is oriented inthe radial direction and in the circumferential direction to form asemicircular shape. The fluid resistance of the auxiliary flow path 212cis composed of a passage configuration resistance and a frictionalresistance of an elbow passage having a square cross section of smallcurvature of about 90 degrees and a substantially right angled bend. Byselecting the passage cross sectional area of the auxiliary flow path212c, it is possible to increase the fluid resistance of this part incomparison with the foregoing embodiment. The downstream wall of theauxiliary flow path 212c against the primary flow is formed by theplanar cover 213 which is fixed to the projecting wall 210d by means ofbolts 214a and 214b. In this embodiment, in the case where an injectoris to be disposed before the throttle valve 3 due to some causes, forexample, the application of a single point injection system, theabove-described arrangement is necessary. In this case, for instance, itmay be the case that the throttle valve shaft is arranged at an angle of45 degrees with respect to the direction in which the molded portion 2cfor holding the hot-wire element is oriented. This is available toreduce the pressure loss as a whole at a high flow rate. The othereffects of the ninth embodiment are the same as those of the firstthrough third embodiments.

FIGS. 22 and 23 show a tenth embodiment of the present invention. Inthis embodiment, it is intended that the auxiliary flow path having arelatively large fluid resistance is formed in the projecting portionhaving a relatively small volume. More specifically, a flow path 222cperpendicular to the auxiliary flow path 222b in which the hot-wireelement is disposed is formed in a doughnat-shape. With such anarrangement, the projecting portion 220d of the body 220 projecting intothe primary flow path 221 is small in comparison with the flow passagelength of the auxiliary flow path 222c. The wall of the auxiliary flowpath 222c on the downstream side against the primary flow is formed by aplanar cover 223 fixed to the projecting portion 220 by a bolt 224 orthe like. The flow resistance of the auxiliary flow path 222c iscomposed of a passage configuration resistance of an elbow having asquare cross section with a relatively high curvature of about 270degrees and a substantially right-angled bend, and a frictionalresistance of the somewhat longer passage length. Except for the casethat the cross section of the auxiliary flow path 222c is extremelyincreased, it is possible to increase the fluid resistance, i.e., theequivalent length of the passage in comparison with the foregoingembodiments. Thus, the arrangement of the tenth embodiment is availablefor an internal combustion engine in which a backblow is large, abackfire is likely to be generated or an intake pulsation is large. Theother effects of the tenth embodiment are the same as those of the firstthrough third embodiments.

FIG. 24 shows an eleventh embodiment of the invention which realizes theauxiliary flow path having a relatively large fluid resistance with astructure in which the axial dimension is not increased. In a probeholder block 230 which is a separate member from a body 240 and iscoupled to a circuit unit 2, the entire auxiliary flow path 242 isformed of an auxiliary flow path 242b in parallel with the primary flowpath 241, an auxiliary flow path 242c having a square cross section anddirected perpendicular to the flow path 242b, an auxiliary flow path242d directed to the upstream side against the primary flow,perpendicular to the auxiliary flow path 242c, and an auxiliary flowpath 242e directed in the radial direction, perpendicular to theauxiliary flow path 242d. The downstream wall of the auxiliary flow path242c relative to the primary flow is formed by a planar cover 243 whichis fixed to the holder block 230 by means of a bolt 244. In thisembodiment, since the length of the auxiliary flow path 242b upstream ofthe hot-wire element 2a is short due to its structure, a mesh member 245is provided at an inlet opening of the body 240. Also, the upstream wallof the holder block 230 relative to the primary flow is extended furtherinto the primary flow relative to the outlet of the auxiliary flow path242e so that the primary flow is prevented from inpinging directly tothe outlet of the auxiliary flow path 242e, thus stabilizing the staticpressure thereat and the flow within the auxiliary flow path to reducethe noises.

In this embodiment, the fluid resistance of the auxiliary flow path 242is composed of a frictional resistance in proportion to the long passagelength and a passage configuration resistance element composed of threeright-angled bends. The equivalent length of the passage the eleventhembodiment is longer than that of the tenth embodiment. In other words,the effect of this embodiment is strong against the backblow, backfireand intake pulsation as in the tenth embodiment. Also, if the fluidresistance of the auxiliary flow path, and in particular, theconfiguration resistance are increased, it is possible to decrease theflow rate distribution ratio of the auxiliary flow path to the primaryflow path at a high flow rate (high speed region). This makes itpossible to reduce the flow rate in the vicinity of the hot-wire elementand is available against the contamination due to adhesion of dust orforeign matters.

In this embodiment, in view of the working formation of the auxiliaryflow path 242, the auxiliary flow path 242 is separately formed from thebody 240 and detachably mounted to the body 240. However, it is apparentthat, if the formation of the auxiliary flow paths 242c and 242e iscarried out by boring from the outside of the body, it is possible toform the auxiliary flow path integrally with the body.

FIG. 25 shows a twelfth embodiment of the invention, in which anauxiliary flow path 252b in parallel to the primary flow path 251 an anauxiliary flow path 252c perpendicular to the auxiliary flow path 252bare formed in a projecting portion 250d of the body 250, and further anoutlet opening 252d of the auxiliary flow path is formed so as to bedirected in the downstream direction of the primary flow with a checkvalve 254. Since the outlet opening 252d is perpendicular to the primaryflow, if the counter flow due to the backblow or backfire is produced,without any modification, the counter flow within the auxiliary flowpath is remarkable in comparison with the foregoing embodiments in whichthe oullet surface of the auxiliary flow path is in parallel with theprimary flow. This is avoided by the check valve 254. The check valve254 made of thin plate material is supported by a retainer 255 that isshort in length than the check valve 254 and fixed thereto by a bolt256. Also, in order to largely hinder the flow from the auxiliary flowpath outlet 252, the check valve 254 is constructed so that it isnormally opened toward the retainer 255 as shown in FIG. 25. When thecounter flow is generated, the dynamic pressure is applied to the checkvalve 255 to thereby clog the auxiliary flow path 252d to prevent thecounter flow from entering into the auxiliary flow path 252.

The fluid resistance of the auxiliary flow path 252 of this embodimentis composed of a passage configuration resistance of the tworight-angled bends and a passage frictional resistance and is smallerthan that of the eleventh embodiment. However, because of the provisionof the check valve, this embodiment is available against the backblow orbackfire. The embodiment is advantageous against the contamination dueto a long service life as described in conjunction with the eleventhembodiment. Incidentally, the auxiliary flow path 252c of thisembodiment is formed in a circular cross section from the outside of thebody 250. Blind plugs 253 and 257 are provided for the respective flowpath formations.

FIG. 26 shows a thirteenth embodiment of the invention. According tothis embodiment, it is possible to attain a simple structure whichincrease the fluid resistance of the auxiliary flow path as in the tenthto twelfth embodiments, that is, which is suitable for an internalcombustion engine in which suffers from a large backblow or backfire orfor an internal combustion engine which generates a large intakepulsation, and which is advantageous against the foreign matter adhesionfor a long time. A throttle 262e is provided downstream of a hot-wireelement 2a of an auxiliary flow path 262b in parallel to a primary flow,formed in a projecting portion 260d of a body 260, thereby reducing across sectional area (diameter) of the auxiliary flow path 262cperpendicular to the primary flow relative to the auxiliary flow path262b parallel to the primary flow. Also, an enlarged portion 262f isprovided before an outlet 262d of the auxiliary flow path 262c, so thatan area of the outlet 262d is equal to that of the inlet 262a of theauxiliary flow path 262b.

By providing the throttle 262e and reducing the diameter of the flowpath 262c to thereby add the passage configuration resistance ofreduction and enlargement, it is possible to increase the fluidresistance of the auxiliary flow path downstream of the hot-wire element2a, in particular, the fluid resistance against the counter flow.Accordingly, it is possible to attain the foregoing effects. Also, thearea of the outlet 262d is increased and the cross sectional area of theflow path 260d is set to the relatively large level, so that it ispossible to reduce the static pressure loss due to the dynamic pressurechange from the inlet to the outlet and it is possible to reduce thepassage friction resistance of the flow path 260d. Thus, the flow ratedistribution ratio in the low flow rate region may be relativelyincreased.

FIGS. 27 to 29 show still another embodiment of the present invention toattain the objects of the invention.

As auxiliary flow path 272 opened to a central portion of a primary flowpath 271 of a projecting portion 270d of a body 270 is defined only byan auxiliary flow path 272b parallel to the primary flow. The surface ofthe projecting portion 270d on the downstream side relative to theprimary flow is made flat. On this surface, there is provided a checkvalve 273 for closing the outlet 272d of the auxiliary flow path whenthe dynamic pressure of the counter flow is applied to that surface. Thecheck valve 273 is backed up by a retainer 274 that has a shorter lengththan that of the check valve 273. The retainer 274 is fixed to theoutlet portion 270d by means of bolts 275 and 276. A circuit unit 282has a long molded portion 272c. A hot-wire element 282a and atemperature compensation element 282b are disposed in the auxiliary flowpath 272b.

Owing to the above-described effect of the check valve, according tothis embodiment, it is possible to realize a hot-wire type flow meterfor an internal combustion engine, with a short axial dimension, inwhich the temperature characteristics are excellent. The flow meter isresistive against the backblow or backfire of the engine. However, inthis arrangement, since the auxiliary flow path 272 has a short passagelength, there are problems such that the reduction effect of thepulsation is small and the flow rate reduction effect in the high flowrate region is not attained.

FIG. 29 shows a partial modification of the embodiment shown in FIG. 27.In this modification, in the auxiliary flow path 292b, a throttle 292eis provided downstream of the hot-wire element 282a whereby it ispossible to reduce the flow rate within the auxiliary flow path 292b inthe high flow rate region and to somewhat damp the pulsation.

The internal combustion engine to which the invention pertains will bedescribed with reference to FIG. 30. FIG. 30 shows a system of theinternal combustion engine provided with an electronic control type fuelinjection unit to which the automotive hot-wire type air flow meteraccording to the invention is applied.

Air for cylinders 500 is sucked through an air filter 503 and is made toflow through a connector pipe 504, a flow meter 1 and an intake manifold501. The flow meter 1 is provided with an auxiliary flow path 22projected into a primary flow path 21. A hot-wire element 2a and atemperature compensation element 2b that is in unison with a circuitunit 2 are provided within the auxiliary flow path 22, thereby detectingthe flow rate of air through this portion to obtain an output relativeto the overall intake air flow rate. A throttle valve 3 for controllingthe intake air flow rate, that is associated with an acceleration pedalof an vehicle is provided in the passage of the flow meter 1.Furthermore, an idle speed control (ISC) valve 8 for controlling a flowrate at the throttle valve fully closed condition (idle speed) isdisposed in the flow meter 1.

On the other hand, fuel is injected into the intake manifold 501 from aninjector 507 by an injection pump 506 coupled to a fuel reservoir 505and is supplied to the engine 500 together with the air.

Into a control unit 510, there are inputted an output signal of thehot-wire element circuit unit 2, a rotational angle signal of thethrottle valve 3, an output signal of an oxygen concentration sensor 508provided in an exhaust manifold 511, an output signal of an engine rpmsensor 509 and the like. Thus, a fuel injection amount and the ISC valveopening degree are calculated. In response to the calculation results,the injector 507, the ISC valve 8 and the like are controlled. Also, adata table of the fuel injection amounts corresponding to the intake airflow rate and the rpm is stored in the control unit 510, so as toimmediately determine the intake air flow rate on the basis of thehot-wire element and the fuel injection amount on the basis of the rpmfrom the rpm sensor, thus controlling the fuel injection amount to beinjected from the injection unit.

We claim:
 1. A hot-wire type air flow meter comprising a primary flowpath constituting an intake air passage of an internal combustionengine, a hot-wire element for measuring intake air, and an auxiliaryflow path provided substantially entirely within said primary flow pathand having mounted therein said hot-wire element, said auxiliary flowpath having an L-shaped configuration including a flow path portionformed in an axial direction of said primary flow path and a flow pathportion formed in a radially inward direction of said primary flow pathand extending at least half way across said primary flow path, andwherein said auxiliary flow path portion in the axial direction of saidprimary flow path is provided eccentrically with respect to said primaryflow path.
 2. A flow meter according to claim 1, wherein said hot-wireelement is provided in the flow path portion formed in the axialdirection of said primary flow path.
 3. The flow meter according toclaim 1, wherein a throttle for throttling air flow is provided at aninlet portion of said auxiliary flow path.
 4. The flow meter accordingto claim 1, wherein a member for forming said primary flow path isintegral with a member for forming said auxiliary flow path.
 5. The flowmeter according to claim 1, wherein the auxiliary flow path portionformed in the radial direction of said primary flow path comprises aplurality of outlet openings at the end thereof opposite said auxiliaryflow path portion formed in the axial direction.
 6. A hot-wire type airflow meter according to claim 1, said auxiliary flow path portiondefined in an axial direction of said primary flow path having a checkvalve for preventing a counter flow back to said auxiliary flow path,said check valve being located in an outlet portion of said auxiliaryflow path.
 7. An internal combustion engine comprising the hot-wire airflow meter; an rpm sensor for sensing an rpm of the internal combustionengine; a fuel injection means for injecting fuel into the sucked air;and a controlling means for determining the corresponding fuel injectionamount on the basis of the sucked air flow rate detected by said hotwire type air flow sensor and the rpm detected by said rpm sensor, andfor outputting a command signal for the determined fuel injection amountto said fuel injection means, wherein said hot-wire type air flow metercomprises a primary flow path constituting an intake air passage of aninternal combustion engine, a hot-wire element for measuring intake air,and an auxiliary flow path provided substantially entirely within saidprimary flow path and having mounted therein said hot-wire element, saidauxiliary flow path having an L-shaped configuration including a flowpath portion formed in an axial direction of said primary flow path anda flow path portion formed in a radially inward direction of saidprimary flow path and extending at least half way across said primaryflow path, and wherein said auxiliary flow path portion in the axialdirection of said primary flow path is provided eccentrically withrespect to said primary flow path.
 8. A hot-wire type air flow metercomprising a hollow body forming a primary flow path constituting anintake air passage of an internal combustion engine; a hot wire elementfor measuring intake air; and an auxiliary flow path formed in a radialarm disposed in said primary flow path within said hollow body andhaving said hot-wire element mounted therein; said radial arm having abore extending therethrough in the direction of said primary flow path,a groove in the downstream surface thereof and communicating with saidbore, and a cover plate secured over said groove to form said grooveinto a channel having an opening into said primary flow path serving asan outlet for said auxiliary flow path.
 9. A hot-wire type air flowmeter as claimed in claim 8, wherein said bore communicates with saidgroove intermediate the ends of said groove and said cover plate coverssaid groove so as to provide a pair of outlets for said auxiliary flowpath at opposite ends of said groove.
 10. A hot-wire type air flow meteras claimed in claim 8, wherein said radial arm extends completely acrosssaid primary flow path within said hollow body.
 11. A hot-wire type airflow meter comprising a hollow body forming a primary flow pathconstituting an intake air passage of an internal combustion engine; ahot wire element for measuring intake air; and an auxiliary flow pathformed in a circumferential projection extending into said primary flowpath within said hollow body and having said hot wire element mountedtherein; said circumferential projection having a bore extendingtherethrough in the direction of the primary flow path, acircumferential groove in the downstream surface thereof andcommunicating with said bore, and a cover plate secured over saiddownstream surface to form said groove into a channel having an openinginto said primary flow path and serving as an outlet for said auxiliaryflow path.